Since the mid-1990""s, there have been a series of dramatic developments for the in-situ treatment of chlorinated solvents. The approach of the present invention is based on the sequential reduction of chlorinated hydrocarbons to innocuous end products such as methane, ethane or ethene. In principal the process has been recognized in scientific circles but, it is just beginning to be investigated for environmental application. The process exploits the use of zero valence state elemental metals to reductively dehalogenate halogenated hydrocarbons. In addition, elemental metals may be used to reduce soluble metals such as chromate to insoluble species (Cr (III)) or metalloids such as arsenic or selenium.
The most common metal being utilized for this purpose is iron. But other metals including tin, zinc, and palladium have also shown to be effective. The process may be best described as anaerobic corrosion of the metal by the chlorinated hydrocarbon. During this process, the hydrocarbon is adsorbed directly to the metal surface where the dehalogenation reactions occur. Increasing surface area (by reducing the size of iron particles) increases the effectiveness of the process.
The variations of the process are complex. Recent research on iron systems indicates three mechanisms at work in the reductive process.
Metallic iron may act as a reductant by supplying electrons directly from the metal surface to the adsorbed halogenated compound.
Metallic iron may act as a catalyst for the reaction of hydrogen with the halogenated hydrocarbon. The hydrogen is produced on the surface of the iron metal as the result of corrosion with water.
Also ferrous iron solubilized from the iron metal due the above reactions may act as a reductant for the dehalogenation of halogenated hydrocarbons.
The rate of the reaction of the metallic iron with halogenated hydrocarbons has been demonstrated to be partially dependent upon the surface area of the metallic iron. As the size of the metallic iron is reduced, surface area goes up as well as chemical reactivity. Initial applications of this technology used iron filings. More recent applications have used iron colloids in the micron size range. The applications of the metallic iron reduction of the present invention incorporate nanoscale colloids. These are colloids that range in size from 1 to 999 nanometers. A colloid of this size may have several advantages in application for in-situ groundwater treatment or for use in above ground treatment reactors. These advantages include:
High surface area with greater reaction kinetics as a result. The increase in kinetics allows for a lower mass loading of iron in the treatment zone or reactor because the residence time required for complete dehalogenation is decreased.
The small size and greater reactivity of the colloid allows for the application of the technology through direct in-situ injection into the subsurface.
The smaller size allows for advective colloidal transport.
The greater reactivity, due to the small size, allows for much lower overall iron mass requirements.
To further enhance the physical and chemical character of the colloid, a metallic catalyst may be used to create a bimetallic colloid. The catalyst further increases the rates of reactions, which further lowers the amount of iron colloid that must be used to create an effective reductive dehalogenation treatment zone in the subsurface or a surface reactor. Metals that may be used as a catalyst with the iron include palladium, platinum, nickel, zinc, and tin.
Production of Nano-Scale Iron Colloids
Introduction
A key limitation on the development of the technology of the present invention is the lack of availability of nanoscale metallic colloids. Research, driven primarily by the materials science needs (hi-tech electronic chips or component industry products), has, over the last decade, contributed to general technologies designed to produce nanoscale colloids. Although, generally the research has been in the area of colloids that are composed of ceramic or other non-metallic inorganic materials and not metal colloids. A significant part of the development effort for the technology of the present invention was the adaptation of the non-metallic nanoscale colloid production methods to the production of metallic nanoscale colloids of the present invention.
The method for the production of metal colloids in the nanoscale range may be divided into two primary approaches:
xe2x80x9cBottom Upxe2x80x9d in which colloids of the appropriate size are produced by being assembled from individual atoms.
xe2x80x9cTop Downxe2x80x9d in which colloids of the appropriate size are produced by attrition of larger existing particles of the metal.
The xe2x80x9cBottom Upxe2x80x9d approach has a greater number of potentially applicable methods, including:
Chemical reduction using sodium borohydride; various soluble metal salts (such as ferrous or ferric chloride for iron) in suspensions of water or various hydrocarbon solvents. This process may or may not be enhanced with sonofication during reaction processes.
Other chemical precipitation reactions in aqueous or hydrocarbon solutions capable of producing metals from soluble salts that may or may not include sonofication during reaction processes.
Various methods of metal volatilization and subsequent deposition, typically under vacuum. These include:
Gas Evaporation
Active Hydrogen-Molten Metal Reactions
Sputtering
Vacuum Evaporation to Running Oil Surface
Evaporation Using Direct Electrical Current Heating
Hybrid Plasmas
The xe2x80x9cTop Downxe2x80x9d approach uses two primary variations of milling or mechanical comminuation, this includes:
Using mechanical agitation of a mixture of the desired colloidal metal, a grinding media, and an organic or aqueous suspension fluid. Examples include ball mills and rod mills.
Systems similar to the above where the mechanical agitation is provided by high-speed gas jets.
Upon searching for a supply of nanoscale colloids the inventors of the present invention found that the only method of production capable of producing nanoscale colloids in large kilogram amounts was the sodium borohydride reduction method. However, this was expensive (up to $5,000 per kilogram) and not practical for full-scale application of the technology.
After an evaluation of other production methods the following determinations were made:
Metal volatilization was also expensive, the reactors available for the production of colloids were limited to kilogram capacities, and the colloids produced are at the lower end of the nanoscale range (typically less than 10 nanometers). With time and further development these technologies may also be applied to the production of nanoscale iron colloids for environmental use.
xe2x80x9cTop Downxe2x80x9d mechanical attrition had the potential of:
Generating colloids of the proper size
Colloid production at a reasonable cost ($100 a kilogram or less)
Production capacity in the 100 to 1000 kilogram range.
However, at the time of the evaluation there was no existing capacity (of any size) for the production of iron colloids using mechanical attrition. All work in the field was being performed on ceramics or other non-metallic inorganic materials. The inventors of the present invention sought a provider of nanoscale ceramic production and generated the specifications and requirements for the production of nanoscale iron colloids.
Production of Nanoscale Colloids by Mechanical Attrition
Nanoscale colloids have been produced in amounts up to 10 kilograms, with scale-up production volumes readily and cost effectively available. The process developed to date includes the following components:
Feed material consisting of approximately  less than 325 mesh sized iron particles.
Organic suspension solvent fluids that:
Have high flash points to prevent explosions; and
Are not reactive to the surface of the iron colloid
Examples may include dodecane, butyl acetate, and polypropylene glycol ethyl ether acetate
Dispersants to act as surface acting agents to prevent the agglomeration of the colloids during the milling process were used.
Examples include SOLSPERSE(copyright) 20,000, SOLSPERSE(copyright) 24,000, SOLSPERSE(copyright) 32,600, SOLSPERSE(copyright) 32,500, DISPERBYK(copyright) 108, DISPERBYK(copyright) 164, and DISPERBYK(copyright) 167.
The materials are placed in a high energy ball milling system that is capable of using grinding media as small as 0.2 mm
Rate of agitation and time of milling are further parameters that are used to control generation of a nanoscale iron colloid of the desired properties. Lower energy milling is used initially to insure proper mixing of the solvent, dispersion, and iron components.
Production Method Effects on Colloid Morphology
Each of the production methods described above produce colloids that have distinct morphology and internal crystal structure. In addition, it is important to recognize that in the nanoscale range quantum size effects begin to become apparent. For example a colloid of 10 nanometer diameter has about 30% of its atoms in grain boundaries (which are highly reactive and subject to quantum effects). These features may have an effect on the physical/chemical behavior of the colloid in use. These effects fall into two broad categories that reflect on production by xe2x80x9cBottom Upxe2x80x9d or xe2x80x9cTop Downxe2x80x9d methods.
A colloid produced by chemical precipitation or reduction, or through the various vapor deposition methods may be nano-structured. This means that the colloid may have nanoscale crystal domains with sharp boundaries between crystals. The grain boundaries are typically only 1 atom thick and there is low dislocation density in the crystal structures.
The reactivity of a colloid of this type may be controlled primarily through the selection of an appropriate overall colloid size and resulting surface area. Smaller size means greater surface area and reactivity; larger size means lower surface area and reactivity.
A colloid produced by mechanical attrition may be nano-crystalline. The crystal domains in the colloid are, relative to the overall colloid size, small. The individual crystal domains are separated by wide amorphous transition regions that exhibit a very high dislocation density. These transition regions may be as large as the crystal domains, but are still termed grain boundaries.
The amorphous transition regions may be highly reactive. The size and intensity of dislocation density of the amorphous boundary regions rather than the absolute size of the colloid may dominate the reactivity of the colloid. A relatively large colloid produced by this method may have the same or greater reactivity than a much smaller colloid produced by xe2x80x9cBottom Upxe2x80x9d methods.
Control of the reactivity of the colloid is a critical feature. The iron undergoes anaerobic corrosion when reacted directly with halogenated solvents or when reacted with water to produce hydrogen. As the reactivity of the colloid increases the hydrogen production rate increases as well. By controlling the rate of hydrogen production using the methods described above, one may design reactive metal colloids with reactivity that will generate hydrogen at the rate required for the desired dehalogenation processes rather than at excessively higher rates (with just water) at which the iron colloid would be consumed (by the water) without reacting with the halogenated solvents undergoing treatment. Control of this type is particularly important for in-situ applications.
Important factors in the control of the colloid morphology using chemical precipitation include:
Concentration of reagents
Variations in the composition of the metal salt used as a feed material
Composition of the suspending solvents
Composition of the reducing agent
Temperature at which the reactions take place
The use and energy of mechanical agitation including sonofication.
Important factors in the control of the colloid morphology using vapor deposition methods include:
Temperature of vaporization reactor
Temperature in the deposition/collector zone
Composition of metal (effects of alloying in addition to elemental composition)
Composition of coating fluids in the collector
Rate of deposition
Important factors in the control of the colloid morphology using mechanical attrition include:
Composition of metal (alloy effects as well elemental composition)
Type and concentration of suspending solvent
Type and concentration of dispersion agent
Size and shape of metal feed stock particles (and concentration in suspension)
Size and shape of grinding media (and amount in suspension)
Energy/rate of ball milling
Time of milling
If gas agitation is used:
Gas composition
Gas pressure, flow rate and configuration of injection system.
Post production processes may also impact colloid morphology and crystal structure, these post production processes include:
Annealing
To various temperatures ranging from room temperature to the melting point of the colloid
At various heating rates and total annealing times
In the presence of various gases
Treatment with other aqueous or organic solutions
Drying processes including:
The use of heat
The use of evaporation
Vacuum drying
The composition of the blanketing gas used during the drying process.
Through the manipulation of the colloid size, morphology, and crystal structure using the above process it is possible to design colloids for variations in specific contaminant types, concentrations, groundwater or reactor flow velocities, subsurface permeability, and provide some control over the transport properties of the colloids during injection.